0 What is keyword?

Keywords are the reserved words  whose meaning has already known to the C compiler . Keywords have standard , predefined meanings in C. So the keywords can be used only for their intended purpose. The keywords cannot be used as variable names because if we do so we are trying to assign a new meaning to the keyword, which is not allowed by the computer. Note that the keywords are all lowercase. Since uppercase and lowercase characters are not equivalent , it is possible to utilize an uppercase keyword as an identifier. Some C compilers allow you to construct variable names that exactly resemble the keywords. However, it is not a good programming practice.  There are only 32 keywords available in C.

      auto         double           int                struct               break
      else          long              switch           case               enum
      register     typedef         char              extern            default
      goto         volatile          if                  while            continue
      sizeof       do                static             float                     for
      signed      void              return           const                 short
      union               unsigned